I've been to vegas, I haven't been to Aruba. I would guess that the deciding factor is if you are looking for relaxation or entertainment. I would assume that if you are going to Aruba you are probably going to an all inclusive beach resort, which would be relaxing and fun.
This is a summary of what to expect in Vegas. Bring comfortable shoes. You will do a lot of walking in the casinos. it's great for gambling (duh). Also every casino has a buffett, prices do vary, but usually they are pretty good. You can find all sorts of entertainment, such as thrill rides, 3d movies, shows, etc. There is shopping everywhere and every casino has a theme. Mgm has the lion habitat which is really cool for a cat lover. You can also go downtown to old vegas and see the light show on the ceiling. Transportation, this has improved, they now have a monorail for which you can buy a pass. They also have a bus service, which you will need to get downtown. There is also trans between some of the casinos. I would reccomend staying on the strip for you first trip. This time of year the weather is probably in the 60s to 70's. It does get cool at night. Your hotel may have an outdoor pool which you can use.